It is necessary, for certain types of equipment, to determine the magnitude of errors introduced as a function of frequency by a circuit employed in that equipment. The circuit is also usually employed over a several amplitude ranges. For example, calibrators for deriving AC calibrating voltages that are applied to AC voltmeters to be calibrated include variable gain amplifiers responsive to a variable frequency AC source. The gain of the amplifier is varied as a function of the range of the AC output signal of the calibrator, while the frequency of the oscillator is varied as a function of the desired output frequency of the calibrator. In the calibrator specifically disclosed in the co-pending application, the amplitude and frequency ranges are respectively between 200 millivolts and 1,100 volts and 10 Hz and 1 MHz.
AC voltmeters incorporate similar variable gain amplifiers designed to be responsive to AC voltages, over wide amplitude and frequency ranges, e.g., between 200 millivolts and 1,100 volts and frequencies between 10 Hz and 1 MHz. Amplifiers employed in AC meters must be capable of scaling the AC voltage applied to input terminals of the meter to a level which can be handled by a precise AC-to-DC detector connected to output terminals of the amplifier. The detector is usually capable of accurately handling voltages only over a relatively narrow range of AC output voltages of the amplifier, for example, 0 to 20 volts. Hence, the amplifier must be capable of providing gain and attenuation of the AC input signal over a wide range of input amplitudes and frequencies.
The variable gain amplifiers employed in calibrators and AC voltmeters employ, as the gain determining elements thereof, resistors which are variable in discrete steps, as a function of the range of the voltage to be handled by the amplifier. The resistors of different amplifiers have different values and are subject to change in values as a function of time, temperature and other environmental factors. The different values of the resistors change the gain of the amplifiers with which they are associated so the output voltages of the amplifiers for a particular amplitude differs from what it is supposed to be. Changing the amplitude of the output voltages of the variable gain amplifiers employed in calibrators and voltmeters thereby causes errors in the amplitude of the calibrating voltage applied by the calibrator to a meter being calibrated and the output reading of the meter.
Amplifiers employed in calibrators and meters for ranging, i.e., scaling, also have variable amplitude versus frequency responses. Each amplifier includes distributed reactances, usually in the form of shunt capacitors tending to make the amplifier output voltage decrease as a function of increasing frequency. To enable accurate AC outputs to be derived by calibrators and accurate readings to be obtained from AC voltmeters, it is necessary to determine the amplitude versus frequency response of the amplifiers and other circuits included in the calibrator and voltmeters.
Numerous switches and complex manual procedures, which are time consuming and subject to error, have been previously used to determine the calibrating errors for different ranges and frequencies of circuits employed in AC calibrators and AC voltmeters. The requirement for numerous switches has introduced errors in the calibration process due to the voltage drops associated with the switches.
It is, accordingly, an object of the present invention to provide a new and improved method of and apparatus for determining calibrating errors of a circuit used in an AC calibrator or an AC voltmeter.
Another object of the invention is to provide a new and improved method of and apparatus for more accurately and more easily determining calibration errors of circuits employed in AC calibrators or AC voltmeters over several AC voltage and frequency ranges.
Another object of the invention is to provide a new and improved meter for providing accurage indications of AC voltage over a wide range of amplitudes and frequencies and calibration errors are compensated in a meter scaling circuit as a function of the meter range setting and frequency of the AC voltage applied to the scaling circuit.